Microscopic Origin of Conducting Channels in Metallic Atomic-Size Contacts

We present a theoretical approach which allows one to determine the number and orbital character of conducting channels in metallic atomic contacts. We show how conducting channels arise from atomic orbitals with a significant contribution to bands around the Fermi level. Our theory predicts that the number of conducting channels with non-negligible transmission is three for Al and five for Nb one-atom contacts, in agreement with recent experiments. These results are shown to be robust with respect to disorder. The experimental values of the channel transmissions lie within the calculated distributions.