  We present first-principles calculations of quantum transport in chemically
doped graphene nanoribbons with a width of up to 4 nm. The presence of boron
and nitrogen impurities is shown to yield resonant backscattering, whose
features are strongly dependent on the symmetry and the width of the ribbon, as
well as the position of the dopants. Full suppression of backscattering is
obtained on the pi-pi* plateau when the impurity preserves the mirror symmetry
of armchair ribbons. Further, an unusual acceptor-donor transition is observed
in zig-zag ribbons. These unconventional doping effects could be used to design
novel types of switching devices.
