it's simple............

current through any conductor is =Avne

now,number of free electrons per unit length=1*A*n

and, momentum of each free electron is=mv.

 

therefore momentum per unit length=Anmv=(I/e)m=I/s        (since.........e/m=s)

 

threfore required answer is I/s as indicated by you.

 

if i am right then rate me.