  Theory predicts that the application of an electric field breaks the
inversion symmetry of AB and BA stacked domains in twisted bilayer graphene,
resulting in the formation of a triangular network of one-dimensional
valley-protected helical states. This two-dimensional network of
one-dimensional states has been observed in several studies, but direct
experimental evidence that the electronic transport in these one-dimensional
states is valley-protected is still lacking. In this study, we report the
existence of the network in small-angle twisted bilayer graphene at room
temperature. Moreover, by analyzing Fourier transforms of atomically resolved
scanning tunnelling microscopy images of minimally twisted bilayer graphene, we
provide convincing experimental evidence that the electronic transport in the
counter-propagating one-dimensional states is indeed valley protected.
