If the length of a conductor wire is doubled by stretching it, keeping the p.d. across it constant, by what factor does the drift velocity of electrons change?

The drift velocity of electrons is given by 
                     v_d= $\frac{\mathrm{eE}}{\mathrm{m}}\tau$
and, electric field E is equal to V/l.

Therefore,         ${\mathrm{v}}_{\mathrm{d}} \propto \frac{1}{\mathrm{l}}$ . 
This implies,
the drift velocity of electrons would reduce to half of it's initial value when the length of the wire is doubled.