Theory of transition from the tunneling regime to point contact in scanning tunneling microscopy

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Abstract
We analyze the transition from the tunneling regime to point contact in scanning tunneling microscopy. The variation of conductance as a function of tip-sample separation is sample and tip specific. Tunneling occurs through an effective barrier even if the potential barrier collapses. Subsequent to the collapse of the effective barrier the point contact is initiated leading to ballistic transport. The ballistic conductance through uniformly increasing contact area exhibits neither sharp quantized steps nor pronounced quantum oscillations. The observed oscillations are explained by the irregular enlargement of the contact area.