Arrive at an expression for drift velocity.

Explanation

We know that drift velocity
`V_d =(eE tau)/m`
We have current
`I="ne" AV_d`…(1)
Substitute `V_d` in eqn.(1)
`I="ne"A(eEtau)/m =("ne"^2 A E tau)/m`
`I/A=("ne"^2 Etau)/m rArr j=("ne"^2 E tau)/m`
We know that
`j=sigmaE`
`therefore sigma cancelE=("ne"^2 cancelE tau)/m`
`sigma=("ne"^2 tau)/m`