Drift Velocity ambiguity in relation

[justwild]

How is it possible that the two relations i.e.,
v=(eE/m)t
and
v=(eE/2m)t
both holds good?
where e is the elementary charge, m is the mass of the elementary particle, E is the electric field, v is the drift velocity and t is the relaxation time.

 

[mfb]

Where do you get these relations?
Maybe they refer to different situations. The first one looks like a uniform acceleration in an electric field, with 0 initial velocity.
The second one might come from a situation where the initial and final velocity have the same magnitude (butopposite direction). Note that the velocity difference is then twice this value, and therefore identical to the first setup.