TrailRenderer.shadowBias

public float shadowBias ;

Description

Apply a shadow bias to prevent self-shadowing artifacts. The specified value is the proportion of the trail width at each segment.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    private TrailRenderer tr;

    void Start()
    {
        tr = GetComponent<TrailRenderer>();
        tr.material = new Material(Shader.Find("Standard"));
        tr.castShadows = true;
        tr.receiveShadows = true;
        tr.shadowBias = 0.3f;
    }
}