  AC conductivity spectra of xAgI-(1-x)AgPO$_3$ fast ion conducting glasses
spanning the flexible, intermediate (isostatically rigid) and stressed rigid
phases are analyzed. The rescaled frequency dependent spectra are mapped into
time-dependent mean square displacements out of which a typical lengthscale
characterizing the spatial extent $\sqrt{\langle R^2(\infty)\rangle}$ of
non-random diffusion paths is computed. The latter quantity is studied as a
function of AgI composition, it is found to display a maximum in the
intermediate phase, providing the first clear evidence of a typical lengthscale
of a dynamical nature when a system becomes isostatically rigid and enters the
intermediate phase.

Disordered Systems and Neural Networks;Materials Science