  The theory of magnetoresistance of the edge state of a two-dimensional
topological insulator is developed. The magnetic field violates the
time-reversal invariance. Magnetoresistance arises due to the energy gap opened
by a magnetic field parallel to the sample surface. The combined action of
impurities and the magnetic field causes backscattering of edge electrons.
Although impurities are necessary for scattering, sufficiently strong
interaction with impurities leads to the suppression of backscattering.
