  We study the influence of electron puddles created by doping of a 2D
topological insulator on its helical edge conductance. A single puddle is
modeled by a quantum dot tunnel-coupled to the helical edge. It may lead to
significant inelastic backscattering within the edge because of the long
electron dwelling time in the dot. We find the resulting correction to the
perfect edge conductance. Generalizing to multiple puddles, we assess the
dependence of the helical edge resistance on temperature and doping level, and
compare it with recent experimental data.
