Phonon-drag induced suppression of the Andreev hole current in   superconducting niobium contacts
  We have investigated how the Andreev-reflection hole current at ballistic point contacts responds to a large bias voltage. Its strong suppression could be explained by the drag excerted by the non-equilibrium phonon wind generated by high-energy electrons flowing through the contact. The hole - phonon interaction leads to scattering lengths of the low-energetic holes down to 100\,nm, thereby destroying the coherent retracing of the electron path by the Andreev-reflected holes. 
Kurt Gloos, Jouko Huupponen, and Elina Tuuli
cond-mat.mes-hall cond-mat.supr-con