public float stretchingStiffness ;



Must be greater than zero and smaller or equal to one.
In many cases, when you want a cloth to be more stiff, actually reducing the number of vertices will get you better results - and performance.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour { void Example() { transform.GetComponent<Cloth>().stretchingStiffness = 1; } }