  Two-dimensional topological insulators are characterized by gapped bulk
states and gapless helical edge states, i.e. time-reversal symmetric edge
states accommodating a pair of counter-propagating electrons. An external
magnetic field breaks the time-reversal symmetry. What happens to the edge
states in this case? In this paper we analyze the edge-state spectrum and
longitudinal conductance in a two-dimensional topological insulator subject to
a quantizing magnetic field. We show that the helical edge states exist also in
this case. The strong magnetic field modifies the group velocities of the
counter-propagating channels which are no longer identical. The helical edge
states with different group velocities are particularly prone to get coupled
via backscattering, which leads to the suppression of the longitudinal edge
magnetoconductance.
