  The presence of strong disorder in graphene nanoribbons yields low-mobility
diffusive transport at high charge densities, whereas a transport gap occurs at
low densities. Here, we investigate the longitudinal and transverse
magnetoresistance of a narrow (60 nm) nanoribbon in a six-terminal Hall bar
geometry. At B= 11 T, quantum Hall plateaux appear at $\sigma_{xy}=\pm2e^2/h$,
$\pm6e^2/h$ and $\pm10e^2/h$, for which the Landau level spacing is larger than
the Landau level broadening. Interestingly, the transport gap does not
disappear in the quantum Hall regime, when the zero-energy Landau level is
present at the charge neutrality point, implying that it cannot originate from
a lateral confinement gap. At high charge densities, the longitudinal and Hall
resistance exhibit reproducible fluctuations, which are most pronounced at the
transition regions between Hall plateaux. Bias-dependent measurements strongly
indicate that these fluctuations can be attributed to phase coherent scattering
in the disordered ribbon.
