  Transport through quantum coherent conductors, like atomic junctions, is
described by the distribution of conduction channels. Information about the
number of channels and their transmission can be extracted from various
sources, such as multiple Andreev reflections, dynamical Coulomb blockade, or
shot noise. We complement this set of methods by introducing the
superconducting excess current as a new tool to continuously extract the
transport channel transmissions of an atomic scale junction in a scanning
tunneling microscope. In conjunction with ab initio simulations, we employ this
technique in atomic aluminum junctions to determine the influence of the
structure adjacent to the contact atoms on the transport properties.
