Hashtable bucket Memory Leak

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C#2.0

C#

Hi,
I am working on finding memory leaks in my application.
I have a hashtable which has elements of type List<IMxCallBack> (where IMxCallBack is native interface).I found that the IMxCallback objects in each list element of hashtable are not disposed.I successfully disposed them and found that there are removed from the heap as well in the leak Dump taken on my application.
But however I found that there are Hashtable objects still on the heap, also the same number of Hashtable+bucket[] objects on heap.But I have disposed the hashtable in dispose as well.
An aaCalConnection obj is created for every click on my window(which switches to another window).Upon click for every 3 seconds the existing aaCalConnection obj is disposed and hence the hashtable as well.And again a new aaCalConnection object is created which inturn creates a new Hashtable.Please find the code below

public aaCalConnection(CONNECTIONTYPE ConnType, string Node, string Application, string Topic, aaDataSubscriptionManager Callback, bool isLocalConnection, aaAccessName accessname, aaCalConnectionManager CalMgr)
        {
            -------------
            ------------
            m_CalbackCache = new Hashtable();
}

void AddCallbackToCache(int datasourceId, IMxCallback callback)
		{
			//Add this callback to the queue of pending writes
			List<IMxCallback> callbackList = m_CalbackCache[datasourceId] as List<IMxCallback>;
			if (callbackList == null)
			{
				callbackList = new List<IMxCallback>();
				m_CalbackCache[datasourceId] = callbackList;
			}
			if (callbackList != null)
			{
				callbackList.Add(callback);
			}
		}

public void RemoveCallbackFromCache(int datasourceId, IMxCallback callback) 
		{
			//Remove this callback from the queue of pending writes
			List<IMxCallback> callbackList = m_CalbackCache[datasourceId] as List<IMxCallback>;
			if (callbackList != null)
			{
				callbackList.Remove(callback);
        		        if (callbackList.Count == 0)
	                	    m_CalbackCache[datasourceId] = null;
		                callbackList = null;
			}
		}

public virtual void Dispose()
        {
			---------------
			----------
			m_CalbackCache.Clear();
			m_CalbackCache = null; 
         }
// Disposing the IMxCallback object which is a userconnection callback object from another class
m_CalConnection.RemoveCallbackFromCache(callback.DataSourceID, callback);
Callback.Dispose();

Now eventhough the cleanoffs are properly done I could see hashtable and HashTable buckets objects on Heap occupying memory.Can anyone tell me why they are not Garbage collected.How should i go about resolving this leak?

Thanks
Satya