Multiple Andreev reflection in single-atom niobium junctions

Single-atom junctions between superconducting niobium leads are produced us ing the mechanically controllable break junction technique. The current-vol tage characteristics of these junctions are analyzed using an exact formula tion for a superconducting quantum point contact. For tunneling between two single atoms with a sufficiently large vacuum barrier, it is found that a single channel dominates the current, and that the current-voltage characte ristic is described by the theory, without adjustable parameters. For a con tact of a single Nb atom it is shown that five conductance channels contrib ute to the conductance, in agreement with the number expected based on the number of valence orbitals for this d metal. For each of the channels the t ransmission probability is obtained from the fits, and the limits of accura cy of these numbers are discussed.