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.