In the presence of a scattering potential, electron transport in a quantum
wire is known to be dramatically modified by backward scattering and unaffected
by forward scattering processes. We show that the scenario is quite different
in Quantum Spin Hall effect edge states coupled at a constriction. The helical
nature of these states leads to the appearance of a forward scattering spin
channel that is absent in other Luttinger liquid realizations. Suitably applied
ac gate voltages can thus operate on the spin of electrons tunneling across the
constriction, and induce in the dc tunneling current a cusp pattern that
represents the signature of the edge state electronic interaction.Comment: 4 pages, 2 Figure