  Atomically precise graphene nanoribbons (GNRs) have emerged as promising
candidates for nanoelectronic applications due to their widely tunable energy
band gaps resulting from lateral quantum confinement and edge effects. Here we
report on the electronic transport characterization of an edge-modified GNR
suspended between the tip of a scanning tunneling microscope (STM) and a
Au(111) substrate. Differential conductance measurements on this
metal-GNR-metal junction reveal loss-less transport properties (inverse decay
length $\beta < 0.001 /\overset{\circ}{\mathrm{A}}$) with high conductance
($\sim 0.1$ G$_0$) at low voltages (50 meV) over long distances ($z > 10$ nm).
The transport behavior is sensitive to the coupling between ribbon and
electrodes, an effect that is rationalized using tight-binding and density
functional theory simulations. From extensive modelling we infer that the
length-independent transport is a manifestation of band transport through
topological valence states, which originate from the zigzag segments on the GNR
edges.
