Size-dependent enhancement of superconductivity in nanowires
  A shape-dependent superconducting resonance can be expected when an energy level associated with the transverse motion in a wire passes through the Fermi surface. We show that the recently observed width-dependent increase of $T_c$ in ${\rm Al}$ and ${\rm Sn}$ nanowires is a consequence of this shape resonance effect. 
A. A. Shanenko, M. D. Croitoru, M. Zgirski, F. M. Peeters, and K.   Arutyunov