Calculate the mean free time \(\tau\) between the collisions for the conduction electron in copper. \(\left[n=8.49 \times 10^{28} \mathrm{~m}^{-3}\right.\), \(\left.ho_{\mathrm{Cu}}=1.68 \times 10^{-8} \Omega \cdot \mathrm{m}\right]\)