  We consider the effects of electron scattering off a quantum magnetic
impurity on the current-voltage characteristics of the helical edge of a
two-dimensional topological insulator. We compute the backscattering
contribution to the current along the edge for a general form of the exchange
interaction matrix and arbitrary value of the magnetic impurity spin. We find
that the differential conductance is a non-monotonous function of the voltage
with several extrema.
