Supercurrent in atomic point contacts and Andreev states

We have measured the supercurrent in aluminum atomic point contacts contain ing a small number of well characterized conduction channels. For most cont acts, the measured supercurrent is adequately described by the opposite con tributions of two thermally populated Andreev bound states per conduction c hannel. However. far contacts containing an almost perfectly transmitted ch annel 0.9 less than or equal to tau less than or equal to 1 the measured su percurrent is higher than expected, a fact that Eve attribute to nonadiabat ic transitions between hound states.