  In contrast to the case of ordinary quantum Hall effect, the resistance of
ballistic helical edge channels in typical quantum spin-Hall experiments is
non-vanishing, additive and poorly quantized. Here we present a simple argument
connecting this qualitative difference with a spin relaxation in the
current/voltage leads in an experimentally relevant multi-terminal bar
geometry. Both the finite lead resistance and the spin relaxation contribute to
a non-vanishing four-terminal edge resistance, explaining poor quantization
quality. We show that corrections to the four-terminal and two-terminal
resistances in the limit of strong spin relaxation are opposite in sign, making
a measurement of the spin relaxation resistance feasible, and estimate the
magnitude of the effect in HgTe-based quantum wells.
