  We investigate the transmission of electrons between conducting nanoribbon
leads oriented at multiples of 60 degrees with respect to one another,
connected either directly or through graphene polygons. A mode-matching
analysis suggests that the transmission at low-energies is sensitive to the
precise way in which the ribbons are joined. Most strikingly, we find that
armchair leads forming 120-degree angles can support either a large
transmission or a highly suppressed transmission, depending on the specific
geometry. Tight-binding calculations demonstrate the effects in detail, and are
also used to study transmission at higher energies as well as for zigzag ribbon
leads.
