The electrical resistivity of single crystals having high residual resistivity ratios was measured down to 1.6K. The temperature dependence below 30K, caused by phonon scattering, exhibited a Tn behaviour, with n = 4.2±0.1 for LaB6. For  samples with a residual resistivity ratio of 160, no transition into the superconducting state was observed down to 5mK.

Electrical Resistivity and Superconductivity of LaB6 and LuB12. Batko, I., Batková, M., Flachbart, K., Filippov, V.B., Paderno, Y.B., Shicevalova, N.Y., Wagner, T.: Journal of Alloys and Compounds, 1995, 217[2], L1-3