Localization and superconducting proximity effect in sandwiched   potassium films
  Thin films of alkali metals when sandwiched at both surfaces by thin metal films loose their conductance. The superconducting proximity effect is used to investigate the change in the alkali film. On the length scale of the film thickness the electronic properties of the alkali film do not change noticeably although its conductance is dramatically reduced, corresponding to localized electrons. 
Manjiang Zhang and Gerd Bergmann