  For the first time, a comparison to the predicted behavior for parallel
critical field is carried out for the model of Kogan and the model of Hara and
Nagai. In this study, thin niobium films in the moderately dirty regime were
considered. Experimental values of the $-C_{2}$ term are seen to be lower than
those from the model of Hara and Nagai. A possible reason for this could be not
including the non spherical Fermi surface of niobium into the model. There is
clearly disagreement with the model of Kogan as the films get cleaner and
thinner, and two films which should be below his critical thickness still show
positive values of $-C_{2}$, in disagreement with his theory.
