Why it is not possible to determine drift velocity of electron by timing its travel through conductor.

The main reason is that electrons are too small to see.

But even if you could see them, a better reason is the fact that

all electrons are identical. So if you spotted one electron on its

way into the conductor, and then start watching the other end to

see when your electron gets there, you would see trillions of trillions

of trillions of electrons pouring out over there, and you'd have no way

to tell which electron is the one you're waiting for.