We introduce simple approximate analytical models for phonon specific heat and ballistic thermal conductance of nanowires. The analytical model is in excellent agreement with the detailed numerical calculations based on the solution of the elastic wave equation and is also in good agreement with the ballistic thermal conductance data by Schwab et al. (Nature 2000, 404, 974). Finally, we propose a demarcating criterion in terms of temperature, dimension, and material properties to capture the dimensional crossover from a three-dimensional (3D) bulk system to a one-dimensional (1D) system.