I got the answer of the hw, but still have question about it. I need to calculate the scattering time μ = eτ / m. τ is the mean time. the unit needs to be in cm^2 / V.S I put the unit like this : e = coulumb τ = s m = kg then it's impossible to get the unit like that. Then I'm wondering if I could use this to find τ and plug it in : σ = ne^2 τ / m then μ = σ / ne , but I got a problem that it's still not possible to solve it since we don't know the material then don't know σ. Who could tell me what's going here. I appreciate it.