  The thermal conductivity \kappa is measured in a series of
La_{2-x}Sr_{x}CuO_{4} (x = 0 - 0.22) single crystals down to 90 mK to elucidate
the evolution of the residual electronic thermal conductivity \kappa_{res},
which probes the extended quasiparticle states in the d-wave gap. We found that
\kappa_{res}/T grows smoothly, except for a 1/8 anomaly, above x = 0.05 and
shows no discontinuity at optimum doping, indicating that the behavior of
\kappa_{res}/T is not governed by the metal-insulator crossover in the normal
state; as a result, \kappa_{res}/T is much larger than what the normal-state
resistivity would suggest in the underdoped region, which highlights the
peculiarities in the low-energy physics in the cuprates.
