The frequency-dependent conductivity of heavy-fermion materials can be expressed by σ σ ( ω ω ) = n e 2 m ∗ ∗ τ τ ∗ ∗ 1 + ω ω 2 τ τ ∗ ∗ 2 , containing the effective mass m ∗ ∗ and the renormalized relaxation rate 1 τ τ ∗ ∗ = m m ∗ ∗ 1 τ τ . Due to the large effective mass, the renormalized relaxation time is also enhanced, leading to a narrow Drude roll-off at very low frequencies compared to normal metals. The lowest such Drude relaxation rate observed in heavy fermions so far, in the low GHz range, was found in UPd 2 Al 3.