  With a reduction in the average grain size in nanostructured films of
elemental Nb, we observe a systematic crossover from metallic to
weakly-insulating behavior. An analysis of the temperature dependence of the
resistivity in the insulating phase clearly indicates the existence of two
distinct activation energies corresponding to inter-granular and intra-granular
mechanisms of transport. While the high temperature behavior is dominated by
grain boundary scattering of the conduction electrons, the effect of
discretization of energy levels due to quantum confinement shows up at low
temperatures. We show that the energy barrier at the grain boundary is
proportional to the width of the largely disordered inter-granular region,
which increases with a decrease in the grain size. For a metal-insulator
transition to occur in nano-Nb due to the opening up of an energy gap at the
grain boundary, the critical grain size is ~ 8nm and the corresponding grain
boundary width is ~ 1.1nm.
