Which is the correct Expression for Drift Velocity of Electrons in a conductor ?

[nishantve1]

i am really really confused . I know what Drift velocity is but the problem is I have three Sources which tell absolutely different expression for drift velocity and I cat relate . please tell which is right .
This website and MIT Profs. Walter Lewin In his Lectures says Vd = eE/m x (Tau)
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmmic.html

And my book says Vd = eE/2m x (Tau)
Thats Ridiculous Heres the proof

What the heck is goin on ?

 

[Darcior]

Actually the second expression is not fully valid...if you take into consideration the time between two successfull collisions of an electron, then the time will be negligible annd the so called relaxation time will tend to 0. So you can't take average velocity in this case and you are bound to take the instantaneous velocity i.e, v=dl/dt.So the second one is not fully correct...by any case...is this book the second part of H.C.Verma ?