  The correlated spin dynamics and the temperature dependence of the
correlation length $\xi(T)$ in two-dimensional quantum ($S=1/2$) Heisenberg
antiferromagnets (2DQHAF) on square lattice are discussed in the light of
experimental results of proton spin lattice relaxation in copper formiate
tetradeuterate (CFTD). In this compound the exchange constant is much smaller
than the one in recently studied 2DQHAF, such as La$_2$CuO$_4$ and
Sr$_2$CuO$_2$Cl$_2$. Thus the spin dynamics can be probed in detail over a
wider temperature range. The NMR relaxation rates turn out in excellent
agreement with a theoretical mode-coupling calculation. The deduced temperature
behavior of $\xi(T)$ is in agreement with high-temperature expansions, quantum
Monte Carlo simulations and the pure quantum self-consistent harmonic
approximation. Contrary to the predictions of the theories based on the
Non-Linear $\sigma$ Model, no evidence of crossover between different quantum
regimes is observed.
