  Elastic backscattering of electrons moving along the helical edge is
prohibited by time-reversal symmetry (TRS). We demonstrate, however, that an
ensemble of magnetic impurities may cause TRS-preserving quasi-elastic
backscattering, resulting in interference effects in the conductance. The
characteristic energy transferred in a backscattering event is suppressed due
to the RKKY interaction of localized spins (the suppression is exponential in
the total number of magnetic impurities). We predict the statistics of
conductance fluctuations to differ from those in the conventional case of a
one-dimensional system with quenched disorder.
