  In this work, the structural and transport properties of
(Nd0.7-xLax)Sr0.3MnO3 manganites with x = 0, 0.1 and 0.2 prepared by solid
state reaction route are studied. These compounds are found to be crystallized
in orthorhombic structural form. The influence of La substitution in place of
Nd at A-site shifts the metal to semiconductor/insulator transition temperature
(TMI) peak towards room temperature with x = 0, 0.1 and 0.2. A composition
prepared with the value of x = 0.2 in (Nd0.7-xLax)0.7Sr0.3MnO3 manganites (i.e.
(Nd0.5La0.2)0.7Sr0.3MnO3), TMI was observed at 289 K which is close to room
temperature. The maximum percentage of TCR values of compounds are increasing
with average radius <r_A> but %TCR are slightly equal in x = 0.1 and 0.2 as
compared to the parent compound. The maximum %TCR value is almost independent
with A-site average radius <r_A> in x = 0.1 and 0.2. The electrical resistivity
data are explored by different theoretical models and it has been concluded
that at low temperature (ferromagnetic metallic region) conduction mechanism
presumably due to the combined effect of electron-electron, electron-phonon and
electron-magnon scattering, while in paramagnetic semiconducting regime, the
variation of resistivity with temperature are explained by (1) Mott variable
range hopping mechanism, (2) Adiabatic small polaron hopping and (3) Thermally
activated hopping. The polaron hopping and thermal activation energies are
decreasing with increase of an average A-site ionic radius (<rA>). An
appropriate enlightenment for the observed behavior is discussed in detail.
