What are Landauer’s conduction channels in an atomic-size metallic contact?

Using microfabricated break junctions, we have produced Al and Au contacts consisting of a small number of atoms. These metallic quantum point contacts accomodate a small number of conduction channels. We have determined the individual transmission coefficients in contacts containing up to six conduction channels. The determination is based on a comparison of the highly nonlinear current–voltage characteristics in the superconducting state with the predictions of the theory for a single channel superconducting contact (in the case of Au contacts superconductivity is induced by means of the proximity effect). We find that in the smallest achievable contacts usually three channels contribute to the transport in the case of Al, whereas only one does in the case of Au. These findings are consistent with the quantum chemistry point of view that the channels are determined by the valence atomic orbitals at the narrowest section of the contacts. c © 1998 Academic Press Limited