Electron mobility is given by
$\mathit{u}=\frac{{\mathit{v}}_{\mathit{d}}}{\mathit{E}}=\frac{{\mathit{v}}_{\mathit{d}}}{\mathit{V}∕\mathit{l}}$
Here $\mathit{E}$ = magnitude of electric field applied ${\mathit{v}}_{\mathit{d}}=$ drift velocity, $\mathit{V}=$ p.d., $\mathit{l}=$ length
So, $\mathit{u}\propto \frac{1}{\mathit{V}}$
Hence, for the constant length and temperature of the conductor, if the potential difference is doubled, the mobility will be halved.