  Electron transport in small graphene nanoribbons is studied by microwave
emulation experiments and tight-binding calculations. In particular, it is
investigated under which conditions a transport gap can be observed. Our
experiments provide evidence that armchair ribbons of width $3m+2$ with integer
$m$ are metallic and otherwise semiconducting, whereas zigzag ribbons are
metallic independent of their width. The contact geometry, defining to which
atoms at the ribbon edges the source and drain leads are attached, has strong
effects on the transport. If leads are attached only to the inner atoms of
zigzag edges, broad transport gaps can be observed in all armchair ribbons as
well as in rhomboid-shaped zigzag ribbons. All experimental results agree
qualitatively with tight-binding calculations using the nonequilibrium Green's
function method.
