AgI is often described as a fast ion conductor. Given that NaCl has around 10⁻⁴ carriers per formula unit immediately below its melting point and that conductivities are ≈ 1 S/cm (AgI) and ≈ 10⁻⁵ S/cm (NaCl), comment on the term fast ion conductor. Assume cell parameters of 5.84 Å for NaCl and 5.06 Å for AgI.