ListBox ItemSource

Hello guys, I'm developing a twitter client using WPF, when the app is loading the ItemSource of a listbox is set (It is set to a IEnumerable collection), I have another method that creates another list and set the ItemSource of the listbox once again.

According to Task Manager This is causing a 10mb memory increase for my app.

Is there any method or technique to dispose all data in ListBox.ItemSource, I've tried setting ItemSource to Null. But this does not make the memory consumption of the app decrease.

Thanks. :)