We examine the change delta G in the two-probe electrical conductance G of a mesoscopic sample due to the switching on of superconductivity and prove that when conductance of the normal mesoscopic host is sufficiently high, the onset of superconductivity always produces a decrease in G. If the superconducting order parameter is of magnitude Delta , we focus attention on the susceptibility chi _Delta=lim_{Delta to 0} delta G( Delta )/ delta ( Delta ²). For weakly disordered, (i.e. ballistic) normal hosts, the average value of this quantity is negative. For diffusive hosts, the mean of chi _Delta increases linearly with (N-G(0)), where N is the number of open channels in the external. current carrying probes, while the fluctuations about the mean are found to be independent of G(0). With increasing normal disorder, chi _Delta undergoes a transition to a region of large fluctuations, before approaching a strongly localized regime, where typical values of chi _Delta are of order (ln²G(0))^-1.