Question

Why is the residual-resistance ratio (RRR) a measure of good crystalline quality?

Answer 1

(The resistivity of a material at room temperature÷ at 0 K) means its ratio. ( since,0 K is not possible so estimation is usually made. it serves as a rough index of the purity and overall quality of a Metal. Since resistivity usually increases as defect prevalence increases, a large RRR is associated with a pure sample. RRR is also important for characterizing certain unusual low temperature states such as the Superconductivity and knodo Effets . [Note that since it is a unitless ratio there is no difference between a residual resistivity ]
...examples metals used in Telephone wires Eg Cu....