  General multi-channel SN junctions fall into two topological classes
depending on whether or not there is a Majorana mode localized at the junction.
This is known to lead to different behaviour of the conductance in the presence
of arbitrary disorder near the junction. We discuss these topological
properties from two perspectives, one based on representing the disorder by a
scattering matrix in series with that of a clean SN junction and one based on
low energy field theory methods. The first approach is used to discuss the
effect of an Ohmic contact between a quantum wire and a three dimensional metal
far from the junction. The second is useful for treating interactions.
