Relationship between the phase and group velocity in a conducting medium

[megaflop]

Is the relation v_{\varphi }v_{g}=v^{2}=\frac{1}{\mu \varepsilon } always true in a plasma ?
Where v_{\varphi }, v_{g} are respectively the phase and group velocity of the electromagnetic wave that is propagating in the plasma.

 

[megaflop]

Is the relation v_{\varphi }v_{g}=v^{2}=\frac{1}{\mu \varepsilon } always true in a plasma ?
Where v_{\varphi }, v_{g} are respectively the phase and group velocity of the electromagnetic wave that is propagating in the plasma.

I said in a plasma but it could be in any conducting medium.