  We experimentally investigate electron transport through the interface
between a superconductor and the edge of a two-dimensional electron system with
band inversion. The interface is realized as a tunnel NbN side contact to a
narrow 8~nm HgTe quantum well. It demonstrates a typical Andreev behavior with
finite conductance within the superconducting gap. Surprisingly, the
conductance is modulated by a number of equally-spaced oscillations. The
oscillations are present only within the superconducting gap and at lowest,
below 1~K, temperatures. The oscillations disappear completely in magnetic
fields, normal to the two-dimensional electron system plane. In contrast, the
oscillations' period is only weakly affected by the highest, up to 14~T,
in-plane oriented magnetic fields. We interpret this behavior as the
interference oscillations in a helical one-dimensional edge channel due to a
proximity with a superconductor.
