Multiple Particle Scattering in Quantum Point Contacts
  Recent experiments performed on weakly pinched quantum point contacts, have shown a resistance that tend to decrease at low source drain voltage. We show that enhanced Coulomb interactions, prompt by the presence of the point contact, may lead to anomalously large multiple-particle scattering at finite bias voltage. These processes tend to decrease at low voltage, and thus may account for the observed reduction of the resistance. We concentrate on the case of a normal point contact, and model it by a spinfull interacting Tomonaga-Luttinger liquid, with a single impurity, connected to non interacting leads. We find that sufficiently strong Coulomb interactions enhance two-electron scattering, so as these dominate the conductance. Our calculation shows that the effective charge, probed by the shot noise of such a system, approaches a value proportional to e* = 2e at sufficiently large backscattering current. This distinctive hallmark may be tested experimentally. We discuss possible applications of this model to experiments conducted on Hall bars. 
Dganit Meidan and Yuval Oreg