  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.
