  We experimentally investigate transport through the side junction between a
niobium superconductor and the mesa edge of a two-dimensional system, realized
in an InAs/GaSb double quantum well with band inversion. We demonstrate, that
different transport regimes can be achieved by variation of the mesa step. We
observe anomalous behavior of Andreev reflection within a finite low-bias
interval, which is invariant for both transport regimes. We connect this
behavior with the transition from retro- (at low biases) to specular (at high
ones) Andreev reflection channels in an InAs/GaSb double quantum well with band
inversion.
