The ionic conductance of which of following ion in fused state is low

$$\begin{gathered} Ionic conduc\tan ce \alpha \frac{1 }{Size of ions } \\ Size in fused state :\hspace{0.17em}L{i}^{+}<N{a}^{+}<K^{+}<R{b}^{+}<C{s}^{+} \\ Ionic conduc\tan ce in fused state :\hspace{0.17em}L{i}^{+}>N{a}^{+}>K^{+}>R{b}^{+}>C{s}^{+} \end{gathered}$$