  The phase fluctuations of the condensate in doped antiferromagnets, described
by a t-t'-J model and a suitable 1/N expansion, provide a mechanism for a
Kosterlitz-Thouless (KT) type of transition to a superconducting state below
T_{c}. In this paper, we present a Monte Carlo study of the corresponding
superfluid weight D_{s}(T) in the classical (large-N) limit, as a function of
temperature and doping. Consistent with generic experimental trends, D_{s}(T)
exhibits a T-linear decrease at low temperatures, with the magnitude of the
slope D_{s}'(0) increasing upon doping. Finite-size scaling in the underdoped
regime predicts values for the dimensionless ratio A=k_{B}T_{c}/D_{s}(0) of
order unity, with A=0.4435(5) in the half-filled-band limit, thus confirming
D_{s}(0) as the fundamental energy scale determining T_{c}. Our Monte Carlo
results for D_{s}(T)/D_{s}(0) vs k_{B}T/D_{s}(0), at 10% hole doping, are found
to be in reasonable agreement with recent measurements on
La_{2-x}Sr_{x}CuO_{4}, with x=0.10, throughout the temperature range below the
theoretical KT transition temperature T_{c}.
