  We report on the observation of strong backscattering of charge carriers in
the quantum Hall regime of polycrystalline graphene grown by chemical vapor
deposition, which alters the accuracy of the Hall resistance quantization. The
temperature and magnetic field dependence of the longitudinal conductivity
exhibits unexpectedly smooth power law behaviors, which are incompatible with a
description in terms of variable range hopping or thermal activation, but
rather suggest the existence of extended or poorly localized states at energies
between Landau levels. Such states could be caused by the high density of line
defects (grain boundaries and wrinkles) that cross the Hall bars, as revealed
by structural characterizations. Numerical calculations confirm that
quasi-one-dimensional extended non-chiral states can form along such line
defects and short-circuit the Hall bar chiral edge states.
