We present a theoretical approach which allows to determine the number andorbital character of the conducting channels in metallic atomic contacts. Weshow how the conducting channels arise from the atomic orbitals having asignificant contribution to the bands around the Fermi level. Our theorypredicts that the number of conducting channels with non negligibletransmission is 3 for Al and 5 for Nb one-atom contacts, in agreement withrecent experiments. These results are shown to be robust with respect todisorder. The experimental values of the channels transmissions lie within thecalculated distributions.
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