The [answer](https://www.quora.com/Why-is-the-ethernet-cable-length-limited-to-300ft) in this post suggests that it has two reasons.

> 
> While the impedance characteristics of a transmission line become less
>  favorable for high speed data as the line gets longer (resistance goes
>  up, capacitance goes up, the line is more susceptible to noise, etc);
>  however, the main reason Ethernet limits cable lengths to 300m is
>  because of time. ... (Explanation of CSMA/CD)
> 
> 
> 

However, modern Ethernet is operated with a switch, which eliminates collision between frames, and a need of CSMA/CD ([CSMA/CD is no longer used recently](https://networkengineering.stackexchange.com/questions/43810/is-the-csma-cd-protocol-active-in-modern-ethernet-networks-that-use-lan-switches))  

For this reason, I think that more general reason is that it is because of impedance of a cable, which makes the signal weaker.  

Which answer is more appropriate between two?