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...ic/ohmmic.html

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


What the heck is goin on ?