  We report simultaneous transport and scanning microwave impedance microscopy
to examine the correlation between transport quantization and filling of the
bulk Landau levels in the quantum Hall regime in gated graphene devices.
Surprisingly, a comparison of these measurements reveals that quantized
transport typically occurs below the complete filling of bulk Landau levels,
when the bulk is still conductive. This result points to a revised
understanding of transport quantization when carriers are accumulated by
gating. We discuss the implications on transport study of the quantum Hall
effect in graphene and related topological states in other two-dimensional
electron systems.
