Conductor resistance calculation method

1. What are the variables that effect on a conductor resistance (I mean all of them)?

First of all I would like to say that I know how to calculate the resistance of a conductor using the method below:

$R=\rho\dfrac{\ell}{A}\,$

where $\ell$ is the length of the conductor, measured in metres [m], $A$ is the cross-section area of the conductor measured in square metres [m²], and $\rho$ is the electrical resistivity of the material, measured in ohm-metres $(\Omega·m)$.

Now I can calculate the resistance of a wire using the relation between the electrical resistivity of the material, the length of the conductor and the cross-section area of the conductor.

But there is a missed variable in the previous method, it's the temperature change $\Delta T$.

2. So I thought that if I would use the previous method with the temperature change variable calculated, then I should multiply it by temperature change in kelvin (because of the direct proportion between resistance and temperature change)? Is that correct?

$R=\rho\dfrac{\ell}{A}\times\Delta T$

In a linear approximation, $$R=R_0(1+\alpha(T-T_0))$$ where $R_0$ is the resistance at temperature $T_0$.