WHAT ARE LANDAUERS CONDUCTION CHANNELS IN AN ATOMIC-SIZE METALLIC CONTACT

Using microfabricated break junctions, we have produced Al and Au cont acts consisting of a small number of atoms. These metallic quantum poi nt contacts accomodate a small number of conduction channels. We have determined the individual transmission coefficients in contacts contai ning up to six conduction channels. The determination is based on a co mparison of the highly nonlinear current-voltage characteristics in th e superconducting state with the predictions of the theory for a singl e channel superconducting contact (in the case of Au contacts supercon ductivity is induced by means of the proximity effect). We find that i n the smallest achievable contacts usually three channels contribute t o the transport in the case of Al, whereas only one does in the case o f Au. These findings are consistent with the quantum chemistry point o f view that the channels are determined by the valence atomic orbitals at the narrowest section of the contacts. (C) 1998 Academic Press Lim ited.