  We combine scanning-tunneling-spectroscopy experiments probing magnetic
impurities on a superconducting surface with a theoretical analysis of the
tunneling processes between (superconducting) tip and substrate. We show that
the current is carried by single-electron tunneling at large tip-substrate
distances and Andreev reflections at smaller distances. The single-electron
current requires relaxation processes between the impurity-induced Shiba bound
state and the quasiparticle continuum, allowing us to extract information on
such relaxation processes from our analysis.
