Which particles are responsible for the conductivity of metals?

$A$ potential difference is applied across the ends of a metallic wire. If the potential difference is doubled,then the drift velocity

In a wire of circular cross-section with radius $r$,free electrons travel with a drift velocity $V$ when a current $I$ flows through the wire. What is the current in another wire of half the radius and of the same material when the drift velocity is $2V$?

Cross-sectional area of a copper wire is equal to the area of a square of length $2 \ mm$. If this copper wire draws $8 \ A$ electric current,then find the drift velocity of free electrons. The number density of electrons in the copper wire is $8 \times 10^{28} \ m^{-3}$.

$A$ potential difference of $V$ is applied at the ends of a copper wire of length $l$ and diameter $d$. On doubling only $d$,drift velocity

The drift velocity of electrons in a conducting wire is of the order of $1 \, mm/s$. However,when the switch is turned on,the bulb glows almost instantaneously. This is because: