Classical electron transport — conductivity and frequency response
Drude (1900): electrons scatter off ion cores with mean free time τ. Equation of motion:
DC conductivity (steady state v̇=0):
AC conductivity (optical response):
Plasma frequency ωₚ²=ne²/mε₀. For ω>ωₚ, metals become transparent. Below ωₚ, light is reflected — that's why metals are shiny.