  We present calculations of electronic quantum transport in graphene
nanoribbons with adsorbed H, F, OH and O, based on a tight binding model
derived from extended Huckel theory. The relaxed atomic geometries of the
adsorbates and graphene are calculated using density functional theory. Our
model includes the effects of the local rehybridization of the graphene from
the sp2 to sp3 electronic structure that occurs when H, F, OH or O bonds
covalently to the graphene. It captures the physics of the scattering
resonances that are induced in the graphene near the Dirac point by the
presence of these adsorbates. We find these Dirac point resonances to play a
dominant role in quantum transport in ribbons with these adsorbates: Even at
low adsorbate concentrations the conductance of the ribbon is strongly
suppressed and a transport gap develops for electron Fermi energies near the
resonance. The transport gap is centered very near the Dirac point energy of
for H, below it for F and OH and above it for O. We predict ribbons with these
adsorbed species under appropriate conditions to exhibit quantized conductance
steps of equal height similar to those that have been observed by Lin et al.
[Phys. Rev. B 78, 161409 (2008)] at moderately low temperatures, even for
ribbons with conductances a few orders of magnitude smaller than 2e2/h.
