  Low-dimensional nanostructured materials can guide light propagating with
very low group velocity vg. However, this slow light is significantly sensitive
to unwanted imperfections in the critical dimensions of the nanostructure. The
backscattering mean free path, xi, the average ballistic propagation length
along the waveguide, quantifies the robustness of slow light against this type
of structural disorder. This figure of merit determines the crossover between
acceptable slow-light transmission affected by minimal scattering losses and a
strong backscattering-induced destructive interference when xi exceeds the
waveguide length L. Here, we calculate the backscattering mean free path for a
topological photonic waveguide for a specific and determined amount of disorder
and, equally relevant, for a fixed value of the group index ng which is the
slowdown factor of the group velocity with respect to the speed of light in
vacuum. These two figures of merit, xi and ng, should be taken into account
when quantifying the robustness of topological and conventional
(non-topological) slow-light transport at the nanoscale. Otherwise, any claim
on a better performance of topological guided light over conventional one is
not justified.
