Resistance of a one-atom contact in the scanning tunneling microscope

The resistance as a function of tip-sample separation in the scanning tunneling microscope is calculated for distances in the transition region between tunneling and point contact. A resistance plateau appears near point contact with value $\frac{A\pi \hslash }{e^2}$, where $A$ is of order unity, its exact value depending on the identity of the tip atom. Good agreement is found with the recent experimental data of Gimzewski and Möller.