Proximity effect and multiple Andreev reflections in gold atomic contacts

We investigate the electronic transport properties of gold point contacts w ith superconducting aluminum leads. The modifications induced by the proxim ity effect in the quasiparticle density of states at the contact region an measured by tunnel spectroscopy. The theory of transport through multiple A ndreev reflections is extended to incorporate these effects and used to det ermine the number and transmission coefficients of the conduction channels in the contact regime. We find that the smallest contacts, formed by one go ld atom between the electrodes, contribute one single channel to the transp ort with variable transmission T between 0.1 and 1.