  The contribution of superconducting fluctuations to the conductivity, or
paraconductivity is studied in the underdoped regime of $La_{2-x}Sr_xCuO_4$
cuprates. A perpendicular magnetic field up to 50 T is applied to suppress the
superconductivity and obtain the normal state resistivity which is then used to
calculate the paraconductivity. Surprisingly enough, it is consistent with a
two-dimensional Aslamazov-Larkin (AL) regime of Gaussian fluctuations close to
the critical temperature. At higher temperature, the paraconductivity shows a
power-law decrease in temperature (as $T^{-\alpha}$) as was previously shown
for underdoped $YBa_2Cu_3O_{7-\delta}$ and $Bi_2Sr_2CaCu_2O_{8+\delta}$
samples. Our observations are not consistent with the existence of
Kosterlitz-Thouless fluctuations. This tends to indicate that the
superconducting pair amplitude is not already defined above $T_C$ in the
pseudogap state.
