  By improving resolution of home-made mutual-inductance measurements
technique, a pronounced step-like structure (with the number of steps n=4 for
all AC fields) has been observed in the temperature dependence of AC
susceptibility in artificially prepared two-dimensional Josephson Junction
Arrays (2D-JJA) of unshunted Nb-AlO_x-Nb junctions with \beta_L(4.2K)=30. Using
a single-plaquette approximation of the overdamped 2D-JJA model, we were able
to successfully fit our data assuming that steps are related to the geometric
properties of the plaquette. The number of steps n corresponds to the number of
flux quanta that can be screened by the maximum critical current of the
junctions. The steps are predicted to manifest themselves in arrays with the
inductance related parameter \beta_L(T) matching a "quantization" condition
\beta_L(0)=2\pi (n+1).
