  We study the shape resonance effect associated with the confined transverse
superconducting modes of a cylindrical nanowire in the clean limit. Results of
numerical investigations of the Bogoliubov-de Gennes equations show significant
deviations of the energy gap parameter from its bulk value with a profound
effect on the transition temperature. The most striking is that the size of the
resonances is found to be by about order of magnitude larger than in ultrathin
metallic films with the same width.
