  In nanocrystalline Nb films, the superconducting Tc decreases with a
reduction in the average particle size below 20nm. We correlate the decrease in
Tc with a reduction in the superconducting energy gap measured by point contact
spectroscopy. Consistent with the Anderson criterion, no superconducting
transition was observed for sizes below 8 nm. We show that the size-dependence
of the superconducting properties in this intermediate coupling Type II
superconductor is governed by changes in the electronic density of states
rather than by phonon softening.
