  In highly resistive superconducting tunnel junctions, excess subgap current
is usually observed and is often attributed to microscopic "pinholes" in the
tunnel barrier. We have studied the subgap current in
superconductor-insulator-superconductor (SIS) and
superconductor-insulator-normal-metal (SIN) junctions. In Al/AlOx/Al junctions,
we observed a decrease of 2 orders of magnitude in the current upon the
transition from the SIS to the SIN regime, where it then matched theory. In
Al/AlOx/Cu junctions, we also observed generic features of coherent diffusive
Andreev transport in a junction with a homogenous barrier. We use the
quasiclassical Keldysh-Green function theory to quantify single- and
two-particle tunneling and find good agreement over 2 orders of magnitude in
transparency. We argue that our observations rule out pinholes as the origin of
the excess current.
