Electron Spin Resonance (ESR) experiments of diluted Nd$^{3+}$ ions in the claimed topological insulator (TI) YBiPt are reported. Powdered samples with grain size from $approx$ 100 $mu$m to $approx$ 2,000 $mu$m were investigated. At low temperatures,
1.6 K $lesssim$ emph{T} $lesssim$ 20 K, the X-band ($9.4$ GHz) ESR spectra show a emph{g}-value of 2.66(4) and a Dysonian resonance lineshape which shows a remarkably unusual temperature, concentration, microwave power and particle size dependence. These results indicate that metallic and insulating behavior coexist within a skin depth of $delta approx$ 15 $mu$m. Furthermore, the Nd$^{3+}$ spin dynamics in YBiPt are consistent with the existence of a emph{phonon-bottleneck process} which allows the energy absorbed by the Nd$^{3+}$ ions at resonance to reach the thermal bath via the conduction electrons in the metallic surface states of YBiPt. These results are discussed in terms of the claimed topological semi-metal properties of YBiPt.
