  Ion conducting $(CaO)_x(SiO_2)_{1-x}$ glasses and melts show a threshold
behaviour in dc conductivity near $x=x_t=0.50$, with conductivities increasing
linearly at $x>x_t$. We show that the behaviour can be traced to a rigid
($x<0.50$) to floppy ($x>0.50$) elastic phase transition near $x=x_t$. In the
floppy phase, conductivity enhancement is traced to increased mobility or
diffusion of $Ca^{2+}$ carriers as the modified network elastically softens.
