We define drift speed of an electron as the average speed of the electron inside the conductor, which is the distance travelled by the electron between two collisions divided by the average collision time or relaxation time.
$$v_d=\frac{s}{\tau}$$
Using the laws of motion for uniform acceleration, we may write the drift velocity to be
$$\mathbf{v_d}\tau=\mathbf{u}\tau+\frac{1}{2}\frac{e\mathbf{E}}{m}\tau^2$$
Now as the average velocity of an electron would be almost zero after any collision, we may say that $\mathbf{u}$ is zero, and hence we may also write
$$\mathbf{v_d}=\frac{1}{2}\frac{e\mathbf{E}}{m}\tau$$
However in Fundamentals of Physics by Resnik and Halliday the drift velocity (which it states is the average velocity of any electron in the conductor) is given as
$$\mathbf{v_d}=\frac{e\mathbf{E}}{m}\tau$$
Meanwhile in Concepts of Physics by prof. H.C.Verma it is given
$$\mathbf{v_d}=\frac{1}{2}\frac{e\mathbf{E}}{m}\tau$$
So which is actually correct?