  In a recent experiment, Young et al. [Nature {\bf 505}, 528 (2014)] observed
a metal to insulator transition as well as transport through helical edge
states in monolayer graphene under a strong, tilted magnetic field. Under such
conditions, the bulk is a magnetic insulator which can exhibit metallic
conduction through helical edges. It was found that two-terminal conductance of
the helical channels deviates from the expected quantized value at
low-temperatures ($=e^2/h$ per edge, at zero temperature). Motivated by this
observation, we study the effect of disorder on the conduction through the edge
channels. We show that, unlike the situation in semiconducting quantum wells, a
disorder Rashba spin-orbit coupling does not lead to backscattering, at least
to leading order. Instead we find the lack of perfect anti-alignment of the
electron spins in the helical channels to be the most likely source for
backscattering arising from scalar (i.e. spin-independent) impurities. The
intrinsic spin-orbit coupling and other time-reversal symmetry breaking and/or
sublattice-parity breaking potentials also lead to (sub-leading) corrections to
the channel conductance.
