Step 1: Relaxation time $\tau$ is the average time interval between two successive collisions of a free electron with the ions/atoms of the conductor.

Step 2: Drift velocity $v_d = \dfrac{eE\tau}{m}$; current $I=neAv_d = \dfrac{ne^2A\tau}{m}E$.

Step 3: Since $E=V/l$: $I = \dfrac{ne^2A\tau}{ml}V \Rightarrow V = \dfrac{ml}{ne^2A\tau}I$.

Step 4: Comparing with $V=IR$: $R = \dfrac{ml}{ne^2\tau A}$.