  We present an experimental study of inverted InAs/GaSb composite quantum
wells in the hybridization regime and contacted by superconducting electrodes.
A front gate is used to vary the Fermi level into the mini-gap, where recent
experiments indicate existence of helical edge modes [arXiv:1105.0137]. Zero
bias dips in differential resistance are observed across the mini-gap,
suggesting transport dominated by Andreev reflection processes. Evolution of
the mini-gap differential resistance with applied bias as well as measured
mini-gap excess current of 150 nA are in good agreement with the prediction of
perfect Andreev reflection of the helical edge modes, which is necessitated by
the absence of back-scattering channels. The perfect Andreev reflection occurs
in spite of a finite barrier at the interface and shows strong sensitivity to
time-reversal breaking - hallmarks of the helical nature of quantum spin Hall
edges.
