Which particles are responsible for the conductivity of metals?

The unit of mobility in terms of fundamental units is . . . . . . .

$A$ straight conductor of uniform area of cross-section carries a current $I$. If $s$ is the specific charge (charge-to-mass ratio) of the electron,what is the total momentum of all the electrons per unit length of the conductor due to their drift velocity?

$A$ copper wire of length $1\, m$ and radius $1\, mm$ is joined in series with an iron wire of length $2\, m$ and radius $3\, mm$ and a current is passed through the wires. The ratio of the current density in the copper and iron wires is

An electric current passes through a wire of non-uniform cross-section,made of a homogeneous and isotropic material. If $j_A$ and $j_B$ are the current densities and $E_A$ and $E_B$ are the electric field intensities at $A$ and $B$ respectively,then $-$

Derive the equation of mobility in terms of relaxation time $(\tau)$. Write its unit.