We study backscattering of electrons and conductance suppression in a helical edge channel in two-dimensional topological insulators with broken axial spin symmetry in the presence of nonmagnetic point defects that create bound states. In this system the tunneling coupling of the edge and bound states results in the formation of composite helical edge states in which all four partners of both Kramers pairs of the conventional helical edge states and bound states are mixed. The backscattering is considered as a result of inelastic two-particle scattering of electrons, which are in these composite states. Within this approach we find that sufficiently strong backscattering occurs even if the defect creates only one energy level. The effect is caused by electron transitions between the composite states with energy near the bound state level. We study the deviation from the quantized conductance due to scattering by a single defect as a function of temperature and Fermi level. The results are generalized to the case of scattering by many different defects with energy levels distributed over the band gap. In this case, the conductance deviation turns out to be quite strong and comparable with experiment even at a sufficiently low density of defects. Interestingly, under certain conditions, the temperature dependence of the conductance deviation becomes very weak over a wide temperature range.
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