Why $V$-$I$ curve for good conductors is not a perfect straight line? I was reading a text from my school textbook it read

"the $V$-$I$ graph for good conductors is not a straight line"

but they didn't explain why. So why a conductor not completely following ohms law is a good conductor?

(The dashed line represents the linear ohm's law. The solid line is voltage V versus current I for a good conductor.)
 A: It is a line for most intents and purposes.
If you dig much deeper there are countless effects all of which make it slightly nonlinear. They could be getting at all sorts of effects.

*

*There is heating due to the conduction, so resistance at high currents will change typically.

*If the junction is not entirely ohmic, e.g. to impurities, it will have slightly less conductivity when weakly biased

*If the current density is extremely large (mesoscopic structures), saturation phenomena can occur.

*At extremely large terminal voltages, the space around the conductor will experience discharges/breakdown leading to additional conductance

A: The most likely reason for the departure from linearity is self heating which  increases the temperature of the metallic? conductor which results in an increase in its resistance.
In this link, Light Bulb Current-Voltage Characteristic, there is described an experiment in which the voltage-current characteristics were found for a light bulb,

and a light bulb with the glass bulb removed and with the filament submerged in water to try and keep its temperature constant,

This seems to show that immersing the filament in water does affect the characteristic.
My only reservation about this experiment is that water is also a conductor and that might have affected the results?
