  We study the amount of backscattering of Valley Hall modes in a classical
topological insulator. In reciprocal systems, the conservation of the valley
index has been argued to be at the root of the high-transmission of Valley Hall
modes, observed in many experimental realisations. Here, we reconsider this
hypothesis by quantitatively analysing the canonical scattering problem of
interface Valley Hall modes impinging on sharp bends which may or may not
conserve the valley index. We consider a tight binding model of graphene
ribbons with an interface and compute the reflection and transmission
coefficients using a transfer matrix formalism. We find that, in all
configurations considered, the transmission of Valley Hall modes is close to
being maximal, even in cases where the valley index is not conserved. Hence
there appears to be no correlation between valley conservation and good
transmission. Our results serve as a reference case for the design of Valley
Hall type metamaterial.
