Current Electricity

The resistivity of a metal is $1 \times 10^{-8} \Omega-\mathrm{m}$. If it contains $9 \times 10^{28}$ electrons per $\mathrm{m}^3$, then the relaxation time of electrons inside the metal is nearly

(electron mass $=9 \times 10^{-31} \mathrm{~kg}$ )